PTL 1 discloses a known example of such a cutting insert.
The cutting insert disclosed in PTL 1 is hexagonal and has acute angle corners and obtuse angle corners that are alternately arranged.
The corner angles of the three acute angle corners are the same, and the corner angles of the three obtuse angle corners are the same.
The cutting insert has such a shape that the projected shape of an upper surface is extended to a lower surface, and a ridge where the upper surface and a side surface intersect and a ridge where the lower surface and the side surface intersect serve as cutting edges.
As illustrated in FIG. 13, with such a shape, all cutting edges of the upper and lower surfaces have the same length L.
The sharpness of a reversible negative cutting insert in which a side surface perpendicularly intersects upper and lower surfaces, such as the cutting insert described in PTL 1, is low. Therefore, as illustrated in FIG. 14, the sharpness is increased by making a major cutting edge be inclined by about 0=5° to 30° with respect to a flat bearing surface (perpendicular to the side surface) that is to be received and supported by an insert support seat of a tool body (and by increasing the axial rake of a cutter by using the inclination).